1. Field of the Invention
The present invention relates generally to optical fiber devices and methods, and in particular to improved systems and methods for providing tunable dispersion compensation in an optical transmission link with minimum differential group delay.
2. Description of Prior Art
An ongoing issue in the development of optical fiber transmission systems is chromatic dispersion. An optical fiber introduces a certain amount of wavelength-dependent time delay into transmitted data. The steepness of the slope of the wavelength-dependence function increases with the length of the transmission line. In today's optical fiber transmission systems, it is possible for an optical fiber to be used to transmit data at different wavelengths over a distance of hundreds, or even thousands, of kilometers. Without suitable dispersion compensation, chromatic dispersion may lead to an unacceptably large difference in arrival times of signals having different wavelengths.
A dispersion compensator addresses the problem of dispersion by introducing offsetting wavelength-dependent delays into the optical transmission link, thus reducing the difference in arrival times to an acceptably small level. One optical device that can be used to provide dispersion compensation is a chirped fiber Bragg grating (CFBG). See, e.g., Eggleton et al., “Integrated Tunable Fiber Gratings for Dispersion Management in High-Bit Rate Systems,” Journal of Lightwave Technology, vol. 18, 1418 et seq. (2000).
A Bragg grating is formed by using a high-intensity ultraviolet light source to “write” a periodic series of changes into the refractive index of a segment of optical fiber. Through the use of a particular writing scheme, it is possible to create a series of refractively modified regions, each of which functions as a wavelength-specific dielectric mirror that reflects light at a particular wavelength back down the length of the fiber segment, while allowing light at other wavelengths to pass through. A “chirped” fiber Bragg grating (CFBG) may be used as a dispersion compensator. In a CFBG, the wavelength reflectivity and spacing of the refractively modified regions are chosen to introduce a negative dispersion in an optical data signal, thereby substantially reducing or eliminating differences in arrival times caused by chromatic dispersion.
A CFBG typically exhibits a certain amount of birefringence. Because of this birefringence, the optical response from a CFBG will exhibit a certain amount of polarization mode dispersion (PMD). The presence of PMD in an optical transmission link will introduce a polarization-dependent delay, causing signals having different polarizations to arrive at different times at the destination point. As used herein, the term “polarization mode dispersion” (or “PMD”) refers to the physical phenomenon that causes the difference in arrival times. The quantification of this phenomenon is referred to herein as “differential group delay” (or “DGD”).
In the simplest case of a grating with dispersion D and birefringence B operating at a given wavelength λ (e.g., 1550 nm), the grating will exhibit first-order PMD, which is characterized by a single value of differential group delay (DGD) between the two principal states of polarization (PSPs) with a value that can be expressed as follows:DGD=BDλ  (1)
It is known that the DGD for a given CFBG can be reduced by connecting thereto an element that introduces a fixed amount of oppositely signed DGD into the transmission link. See, e.g., U.S. Pat. No. 6,137,924 to Strasser et al. (hereinafter referred to as “the '924 patent”), which discloses a technique for eliminating DGD by adding into an optical path to a static CFBG a segment of polarization-maintaining fiber (PMF). The amount of DGD introduced by the PMF segment varies with its length.
Further known in the art is a CFBG that is tunable over a range of dispersion values. Tuning is accomplished in a number of ways including, for example, applying to the CFBG a thermal or strain component, the gradient of which can be precisely controlled. Moreover, it is known to concatenate two or more CFBGs to achieve a larger or more symmetric tuning range.
Current techniques, such as those disclosed in the '924 patent, are directed to the use of a fixed DGD element in conjunction with a static (i.e., non-tunable) CFBG. While these techniques are attractive because of their simplicity, it is not known how to apply them to a dispersion compensator having one or more tunable gratings. In a tunable CFBG, DGD varies as a function of the tuned dispersion. Thus, where a fixed DGD element is used to provide PMD compensation in conjunction with a tunable grating, it will be appreciated that the fixed DGD element can at best only minimize the DGD for one particular setting of a tunable CFBG. Some DGD will always result at other settings. As a result, there exists a need for a means of achieving the minimum value of DGD in a CFBG over its entire tuning range.
A further issue arises where two or more gratings are concatenated. In that case, there are many tuning voltages that will yield the same value for total dispersion. Thus, an effective DGD minimization technique must allow for different tuning trajectories. Also, a particular tuning trajectory may be dictated, for example, by considerations of power consumption, or the like. In this case, a means of constructing the device in order to achieve minimum DGD along a given tuning trajectory is needed.